In motor vehicles employing a centrifugal-type air blower in the air system for the vehicle's occupant compartment, those foreign particles heavier than air such as snow, water and dirt drawn into the system from the outside tend to gravitate radially outward toward the outside wall of the blower's scroll housing and unless prevented could be borne by the air onto the windshield in the defrost mode. Normally, this is prevented in the system ducting by employing a large intervening plenum between the blower and the windshield to trap such airborne particles. But the space such plenum requires can be difficult to allocate without intruding on other needed space particularly with the down-sizing of passenger cars.